To produce well controlled laser pulses in a laser cavity containing a gain medium with a long upper state lifetime, such as a Nd:YAG laser, an electro-optic Q-switch is often employed. One of the most commonly used crystal materials used for Q-switches is Lithium Niobate (LiNbO3, hereafter LN) because it is commercially available in large sizes with good optical quality, making it a cost-effective choice.
An electro-optic Q-switch is placed inside the laser cavity and uses the Pockels effect in the crystal to change its refractive index with an applied voltage. A common implementation for a LN Q-switch has the laser mode propagating along the crystallographic Z-axis and applies the voltage along the X-faces of the Q-switch.
A laser relies on the optical field in the laser mode to stimulate emission of the upper level states in the gain medium. The Q-switch in the laser cavity is for spoiling (reducing) the Q-factor of the laser cavity, defined as the ratio of energy circulating in the laser mode to the energy lost per cavity round trip. In a Z-cut LN Q-switch, a voltage applied will induce a birefringence that makes the LN crystal act as a quarter-wave plate (or half-wave plate depending on the cavity design, i.e. other polarization elements), introducing loss and spoiling the Q-factor.
A typical sequence of events for generating a laser pulse starts with the voltage being applied to the Q-switch, followed by pumping the gain medium, increasing the density of upper state levels. Lasing will be prevented at that stage because the Q-switch induces enough loss to prevent the laser mode from building up energy. Once the gain medium is sufficiently pumped, the voltage applied to the Q-switch is eliminated. This removes the voltage-induced birefringence in the LN crystal, the laser cavity loss is reduced, increasing the Q-factor, and the laser pulse builds up to extract most of the stored energy in the upper energy level. A good Q-switch is characterized by a high loss induced under voltage and low insertion loss under a no-voltage condition.
However, LN is known for building up pyro-electric charges when undergoing a temperature change. These charges appear on the crystallographic Z-faces, which coincide with the optical faces in the typical LN Q-switch implementation. This can induce non-uniform internal electrical fields with field lines terminating on the electrical contacts. These fields lead to index of refraction changes. If the charges move around on the surface, the local non-uniformity in refractive index can become even higher.
The laser operation is affected by these pyro-charge induced electric fields in two ways: Under a no-voltage condition, the induced birefringence leads to some loss, lowering the total pulse energy. More importantly, particularly for high gain laser systems, the charges will adversely affect the hold-off under applied voltage. The charges lead to deviations away from the desired performance of a uniform wave plate. The LN crystal will not induce as much loss as desired, and this may lead to premature lasing before the voltage is switched off. This is typically called pre-lasing and in extreme cases leads to multiple laser pulses during the pumping phase of the gain medium. As a consequence, the pulse energy is poorly controlled and typically much lower than desired, and the timing of the pulses is also not well controlled.
The low intrinsic electrical conductivity of LN of about 1×10−18 ohm−1·cm−1 at room temperature combined with LN's natural pyro-electric response can lead to significant static electrical surface charging following temperature changes to the LN crystal. The surface charge build-up can be ameliorated by various different approaches to dissipate the pyro-electric charges once they are produced by a temperature change.
Most disclosed approaches for enhancing charge dissipation for LN crystals rely on increasing the electrical conductivity of the LN crystal from its low intrinsic electrical conductivity level so that the electrical charges can more quickly dissipate. For example, doping has been investigated as to its effect on increasing the electrical conductivity of LN, but has not by itself yielded the required electrical conductivity increase. UV illumination has also been shown to increase the electrical conductivity of LN.